Monoamine oxidases A and B are localized in the outer mitochondrial membrane. Each enzyme contains an 8 alpha-S- cysteinyl FAD coenzyme and catlyzes the oxidative deamination of biogenic amines such as serotonin and dopamine which function as neurotransmitters and also functio in regulation of renal sodium reabsorption. This project seeks to achieve an understanding of their detailed catalytic mechanisms; particularly the mode of C-H bond cleavage and to use structure-functon studies to elucidate the factors important in substrate specificities for their respective catalytic sites. Using a novel yeast expression system for the human liver enzymes, the influence of riboflavin analougue structure on; a) the covalent flavin binding to the enzymes and b) the function of the coenzyme in catalysis will be investigated. Mechanistic approaches including stopped flow kinetic studies, kinetic isotope effect studies, and the influence of magnetic field on the kinetic properties will be employed to distinguish among three proposed catalytic mechanisms in the literature as well as to test whether MAO A and MAO B function via similar catalytic mechanisms. The results from these studies should provide new insights into the structures of the active sites of these two pharmacologically import enzymes which may lead to the development of improved specific inhibitors which may be useful in the clinical treatment of disorders such as depression and to potentiate L-DOPA therapy of patients with Parkinson's Disease.